usually there's two part to a perc. The perc and the ice catcher above it (3 little bumps to keep ice cubes in place)

a perc essentially slows the air/smoke down by drawing it to a narrow point (the laser slits) and then expanding it back out to the uptube. This allows the smoke to thicken giving a stronger hit.

because cold air sinks, the air beneath the ice will collect and thicken thus cooling the smoke and slowing it down further.

Also, because the perc loops downward, it will act as a second bubble bong as the cold, melted ice collects.

Happy smoking!